`PieDataLabel.distance` does not support a distance of `0`

[hcpchris]

As identified in a comment to #183 , the setter on PieDataLabel.distance is a little too-broad in validating False-y values. Because it uses the if not value: pattern, that prevents the user from being able to set PieDataLabel.distance = 0. This should be fixed to be more precise (i.e. if value is None:)